A quick summary of progression through the qualifying session - the rank ordered progression chart:
And the time-based progression chart which shows a little more clearly how times are bunching up:
Here are the original session utilisation charts, showing the times recorded by each according to elapsed session time. Firstly with green and purple times measured across all three parts of qualifying:
Secondly, with green an purple times relative to each session:
There is, however, a problem with all my laptime based analyses - they include times that were deleted:
So in an alternate universe where there were no deleted times, here's how cutoff times evolved:
Details of how these charts were created can be found in the Wrangling F1 Data With R Leanpub book.